This invention was made in the course of, or under, a contract with the United States Energy Research and Development Administration. It relates generally to a novel iridium base alloy composition and particularly to an alloy suited for use as an encapsulation material for radioisotope fuels. Radioisotope fuels have found considerable use as both terrestrial and space power sources. Such fuels utilize an isotope which is an alpha, beta, or gamma emitter. Heat is produced from these nuclear emissions and converted into electrical energy by means of thermoelectric generators or thermionic or dynamic converters.
The most prominent radioisotope fuels at present are .sup.238 PuO.sub.2 and .sup.244 Cm.sub.2 O.sub.3. These particular isotopes in the oxide form are desirable because of their refractory properties. The .sup.238 PuO.sub.2 and .sup.244 Cm.sub.2 O.sub.3 are generally sintered into spherical balls or cylindrical pellets.
Radioisotopic fuels which are used in space power systems must be encapsulated in a highly reliable material, not only to contain the fuel for normal operation for several years, but to survive launch abort situations, severe aerodynamic heating on re-entry and high velocity impact after years of high temperature service. Various alloys have been developed for use as an encapsulation material in this type of environment. See for example commonly assigned U.S. Pat. Nos. 3,737,309, 3,918,965 and 3,970,450. The most prominent encapsulation alloys have been iridium or iridium-tungsten, each sometimes containing ppm levels of various dopants.
Of particular recent interest has been the alloy described in commonly assigned U.S. Pat. No. 3,970,450. This alloy comprises an iridium matrix containing 20-50 ppm aluminum, 20-100 ppm iron, 5-20 ppm nickel, 50-100 ppm rhodium and 15-50 ppm thorium, and in some cases 0.3 wt. % tungsten. While this alloy exhibits higher tensile strength, greater impact elongation and a lesser tendency for grain growth than unalloyed Ir or Ir-0.3% W, its impact resistance drops considerably at temperatures below about 1250.degree. C., and its ductility is significantly reduced after exposure to high temperatures for extended periods. For space nuclear power systems it is very important that fuel encapsulation materials be resistant to long term high temperature conditions, e.g., 1330.degree. C. for several years, as well as brief excursions at higher temperatures, e.g., 1800.degree. C., without excessive loss of impact properties.